As used herein, the terms "support" and "carrier" are interchangable. Metallocene is defined as a derivative of a cyclopentadienylide (Cp), which is a metal derivative containing at least one cyclopentadienyl moiety and a transition metal. The transition metal is selected from Groups IV-B, V-B and VI-B metals, preferably IV-B and V-B metals, preferably titanium, zirconium, hafnium, chromium or vanadium; most preferable Zr, Hf and Ti. The catalyst system to be placed on a support may also contain both Cp and non-Cp transition metal groups.
Homogeneous or non-supported metallocene and organoaluminum catalyst systems are known and exploited for their high catalytic activity when employed in olefin polymerization and for their ability to produce polymers with terminal unsaturation. However, these homogeneous catalyst systems suffer from the limiting disadvantage of producing polymer which sticks to the reactor walls during the polymerization process or polymer having small particle size and low bulk density which limit their commercial utility. Typically polymer particle size and bulk density are determined by the morphological properties of the catalyst solid component [i.e., an inert carrier or support media]. Absent a solid component in the polymerization media, inferior particle size of the final polymer product results. Obtaining a useful supported catalyst for metallocene/alumoxane systems has been a problem in the past. Likewise, maintaining commercially acceptable levels of catalyst activity with minimal levels of reactor fouling occurring during polymerization, is also a problem.
Methods known in the art to obtain polymer product having uniform, compact spherical particles, narrow particle size distribution and/or high bulk density, include: (1) preactivating or precontacting the metallocene and organoaluminum (EPA 302,424, EPA 354,893), (2) prepolymerizing a heterogeneous catalyst system in the presence of at least one olefin (EPA 426,646, U.S. Pat. No. 4,871,705); (3) utilizing finally divided alumoxane to yield a powdery polymer, (EPA 279,586); and (4) utilizing a supported catalyst system and fillers in the catalyst system with affinities for polyolefins (EPA 314,797). Although meeting their objective, these techniques suffer due to either unacceptable levels of reactor fouling which occur during polymerization, low catalyst activity or producing a polymer having too small particles or broad molecular weight.
Various techniques for making supported catalyst systems known in the art include the following. Chang describes various methods for preparing heterogeneous catalyst using hydrated carriers or a wet monomer (EPA 367,503, EPA 336,593, U.S. Pat. No. 4,912,075). Chang's hydrated carrier techniques to produce a supported catalyst are limited by the amount of water added to the carrier, since that determines the amount of methylalumoxane (MAO) which can be placed on the support. The activity is acceptable for the polymerization of ethylene, but not for the polymerization of propylene monomers. Meanwhile, Japanese Kokai number SHO 61 [1986]--108,610, discloses dehydration of the support in preparation of the heterogeneous catalyst, by baking the carrier at a temperature of about 500.degree.-800.degree. C. It is disclosed that if the temperature rises about 800.degree. C., sintering is induced which leads to destruction of micropores. In U.S. Pat. No. 4,659,658 Coleman et al., describe a combination of supported titanium compounds wherein one titanium component, is supported on a magnesium dichloride carrier while the other, a metallocene component, is supported on alumina. Welborn describes in U.S. Pat. No. 4,808,561 and U.S. Pat. No. 4,701,432 techniques to form a supported catalyst where the inert carrier, typically silica, is calcined, and contacted with a metallocene(s) and an activator/cocatalyst component. The metallocene and activator may be contacted simultaneously or in sequence and deposited onto the carrier to achieve the heterogeneous system. U.S. Pat. No. 4,808,561 discloses that the order of addition of metallocene and cocatalyst onto the support material can vary and is unimportant in achieving a catalytically active supported catalyst system. The preferred embodiment section of U.S. Pat. No. 4,808,561, however, discloses that optimum results are obtained when the alumoxane is dissolved in a suitable inert hydrocarbon solvent which is first added to the dehydrated support material and, slurried in the same or other suitable hydrocarbon liquid. The metallocene is added to the slurry thereafter. The prior art methods described above for producing supported catalysts systems have concentrated on (1) varying treatments of the support or carrier or (2) varying the manner of addition of the catalyst onto the support.
While not wishing to be bound by theory, the Inventors believe that the low catalytic activity and reactor wall fouling, which occurs during polymerization, is due to several factors. First, residual solvent remains in the pores of the support material employed at the stage after placement of the catalyst onto the carrier. The residual solvent prevents the catalyst system from securely anchoring itself onto the carrier or into the pores of the carrier. Thus when the supported catalyst is added to the reaction polymerization vessel, the catalyst disassociates from the support, and migrates to the reactor walls where monomer can polymerize therefrom and cause fouling. Secondly, when methyl alumoxane (MAO) is used as cocatalyst in the polymerization at temperatures about or greater than 40.degree. C., the MAO dissolves and extracts the metallocene catalyst from the support and forms a soluble catalyst in the polymerization medium. This soluble catalyst easily deposits polymer onto the reactor walls and/or generates very small particles of low bulk density which are undesirable in a commercial reactor.
Although little has been written regarding commercially marketable catalyst, those skilled in the art are aware that stability, storage and use of transportation of a supported catalyst are major concerns with regard to a commercially marketable catalyst. Vendors prefer catalysts with catalytic stability of about 6 months or more. Few is any catalyst systems are known which when placed on a support address these commercial needs.
The art, as yet, lacks a method to address these problems raised by the prior art techniques for supporting and maintaining a catalyst with high activity, and stability, and yet reduce fouling to commercially acceptable levels during the polymerization reactor process.